As the size of functional elements in integrated circuits decreases, complexity and interconnectivity increases. To accommodate the growing demand of interconnections in modem integrated circuits, on-chip interconnections have been developed. Such interconnections generally consist of multiple layers of metallic conductor lines embedded in a low dielectric constant material. The dielectric constant in such material has a very important influence on the performance of an integrated circuit. Materials having low dielectric constants (i.e., below 2.5) are desirable because they allow faster signal velocity and shorter cycle times. In general, low dielectric constant materials reduce capacitive effects in integrated circuits, which frequently leads to less cross talk between conductor lines, and allows for lower voltages to drive integrated circuits.
Low dielectric constant materials can be characterized as predominantly inorganic or organic. Inorganic oxides often have dielectric constants between 2.5 and 4, which tends to become problematic when device features in integrated circuits are smaller than 1 .mu.m. Organic polymers include epoxy networks, cyanate ester resins, poly(arylene ethers), and polyimides. Epoxy networks frequently show disadvantageously high dielectric constants at about 3.8-4.5. Cyanate ester resins have relatively low dielectric constants between approximately 2.5-3.7, but tend to be rather brittle, thereby limiting their utility. Polyimides and poly(arylene ethers), have shown many advantageous properties including high thermal stability, ease of processing, low stress/TCE, low dielectric constant and high resistance, and such polymers are therefore frequently used as alternative low dielectric constant polymers.
The dielectric constant of many materials can be lowered by introducing air (voids) to produce nanoporous materials. Since air has a dielectric constant of about 1.0, a major goal is to reduce the dielectric constant of nanoporous materials down towards a theoretical limit of 1. Several approaches are known in the art for fabricating nanoporous materials. In one approach, small hollow glass spheres are introduced into a material. Examples are given in U.S. Pat. No. 5,458,709 to Kamezaki and U.S. Pat. No. 5,593,526 to Yokouchi. However, the use of small, hollow glass spheres is typically limited to inorganic silicon-containing polymers.
In another approach, a thermostable polymer is blended with a thermolabile (thermally decomposable) polymer. The blended mixture is then crosslinked and the thermolabile portion thermolyzed. Examples are set forth in U.S. Pat. No. 5,776,990 to Hedrick et al. Alternatively, thermostable blocks and thermostable blocks alternate in a single block copolymer, or thermostable blocks and thermostable blocks carrying thermostable portions are mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermostable blocks. Dielectrics with k-values of 2.5, or less have been produced employing thermostable portions. However, many difficulties are encountered utilizing mixtures of thermostable and thermostable polymers. For example, in some cases distribution and pore size of the nanovoids are difficult to control. In addition, the temperature difference between thermal decomposition of the thermolabile group and the glass transition temperature (Tg) of the dielectric is relatively low. Still further, an increase in the concentration of thermostable portions in a dielectric generally results in a decrease in mechanical stability.
In a further approach, a polymer is formed from a first solution in the presence of microdroplets of a second solution, where the second solution is essentially immiscible with the first solution. During polymerization, microdroplets are entrapped in the forming polymeric matrix. After polymerization, the microdroplets of the second solution are evaporated by heating the polymer to a temperature above the boiling point of the second solution, thereby leaving nanovoids in the polymer. However, generating nanovoids by evaporation of microdroplets suffers from several disadvantages. Evaporation of fluids from polymeric structures tends to be an incomplete process that may lead to undesired out-gassing, and potential retention of moisture. Furthermore, many solvents have a relatively high vapor pressure, and methods using such solvents therefore require additional heating or vacuum treatment to completely remove such solvents. Moreover, employing microdroplets to generate nanovoids often allows little control over pore size and pore distribution.
In yet another approach, U.S. Pat. No. 5,744,399 to Rostoker et al., a low dielectric constant layer is formed by fabricating a composite layer that contains one or more fullerenes and one or more matrix forming materials. The fullerenes may thereby remain in the matrix, or be removed from the matrix to produce a nanoporous material. The introduction of voids by employing fullerenes, however, has several disadvantages. For example, the molecular species of fullerenes exists only in a relatively limited size range from 32 to about 960 carbon atoms (or heteroatoms). Furthermore, the production of fullerenes, and isolation of fullerenes in a desired molecular size may incur additional cost, especially when needed in bulk quantities. Moreover, fullerenes are typically limited to a spherical shape.
Although various methods of producing nanoporous materials are know in the art, all or almost all of them suffer from one or more disadvantages. Therefore, there is a need to provide improved methods and compositions to produce nanoporous low dielectric material.